Magma and Lava

Magma is defined as molten or partially molten rock within or under the crust of the Earth. It consists of a mixture of molten rock, suspended crystals and gas. Some of the  gases found in magma are carbon monoxide, ammonia and carbon dioxide.

Figure 1.1. The source rock constitution as well as the level of partial melting decide the composition of primary magmas.

Figure 1.1. The source rock constitution as well as the level of partial melting decide the composition of primary magmas.

When volcanoes erupt, magma is extruded onto the Earth’s surface as lava and igneous rocks (“fire formed”) are developed. In terms of modes of their appearance, igneous rocks can be intrusive (plutonic) or extrusive (volcanic).

Intrusive or plutonic rocks form when magma cools and crystallizes within the planet’s crust. Surrounded by the pre-existing rock, this magma cools gradually, forming coarse grained rocks.

Extrusive or volcanic rocks form the when magma is extruded onto the Earth’s surface as lava and pyroclastic materials {Greek pyro (fire) and klastos (broken)}. These rocks cool and solidify more rapidly than intrusive igneous rocks forming fine grained rocks.

Due to the molten rock component, nearly all magmas are essentially liquids whose composition, temperature and viscosity vary considerably depending on the character of the volcanic eruption and the tectonic site of the volcano. Accumulation of magma within the lithosphere takes place in magma chambers, or reservoirs. Magma remains in a magma reservoir until it (a) travels into a different reservoir, (b) cools forming igneous rocks or (c) erupts as a volcano.

Magma in Kamoamoa Ahupua`a, Hawaii.

Magma in Kamoamoa Ahupua`a, Hawaii.
Photo credit: Paul Bica

Magma composition is commonly correlated with environments of magma formation. Even though a vast variety of magmas exist on the planet, magmas are a result of the creation of primary magmas termed basalts. Partial melting of the Earth’s mantle and crustal rock melting, regardless of the tectonic setting, will give rise to primary magmas. Production of basaltic magmas will indirectly culminate in the wide variety of magmas found in different tectonic sites. Basalts are distinguished by their low viscosity, high density, low volatile contents and high temperature. When the crust melts, however, the resulting magma is more silicic, lower in density and cooler with higher level of viscosity and volatile compounds, mainly water.

During partial melting, under particular conditions of temperature and pressure, minerals with low melting point will liquefy, while part of the crustal or mantle source rock will remain solid. Mantle and crustal source rocks are composed mainly of silicate minerals, whose chemical components, and their proportion in the melt will characterize the outcome of the primary magma mixture. Mantle-originated magma erupts in all tectonic environments, remarkably at oceanic islands, mid-ocean ridges, continental and oceanic arcs and continental rifts.

Composition of primary basaltic magmas varies depending on the nature of the source mantle which varies with tectonic setting (Figure 1.1). The conditions of pressure and temperature at which partial melting occurs, also dependent on tectonic setting, are different. They also influence the composition of the resulting primary melts, since the composition of the source rocks and the conditions of melting are different in each case. The chemical composition of magma, including the presence of volatiles, will strongly influence its physical properties, specifically its rheology and the nature of eruption.

Magma and Lava: Composition

Silicates are the most abundant minerals comprising the Earth’s crust. When crystal rocks melt and form magma, the magma is typically abundant in silica. Thus, virtually all magmas, with some exceptions, such as carbonatite magmas, contain high contents of silica ranging between 45% and 77%. Besides silica, other major elements of magma include calcium, aluminum, magnesium, iron, sodium, potassium, titanium in addition to smaller quantities of many other elements.

The compositional diversity of magma depends on variations of the mixture of source rocks being melted, differences in the quantity of origin rocks, crystallization of magmas during its rise and storage in the crust, variations in the quantity of volatile compounds in the source areas, mixing with other magmas, and contamination by (due to exposure to) country rocks.

Although silica is notably, the main component of virtually all magmas, the percentage of silica varies, serving to distinguish four broad categories of magma (Table 1.1) that depend on their distinctive compositions: Silicic or felsic (>63% SiO2), intermediate (52–63% SiO2), basic or mafic (45–52% SiO2) and ultrabasic or ultramafic (<45% SiO2).

    Table 1.1. Division of magmas according to composition.

Table 1.1. Division of magmas according to composition.

Silicic Magma

Silicic or felsic magma contain >63% SiO2 in addition to substantial quantities of sodium, potassium and aluminum. Silicic magma has low percentages of iron, magnesium and calcium.

    Granite is an igneous rock with felsic (silica-rich) composition.

Granite is an igneous rock with felsic (silica-rich) composition.

This magma is characterized by eruption temperatures of <900°C (1,652 °F) and high viscosity. Cooling of silicic magma produces igneous rocks, for example rhyolite and granite. Characteristic silicic magma rocks include quartz, muscovite and orthoclase.

Intermediate Magma

Containing 52-63% SiO2, intermediate magmas are situated in the category between silicic/felsic and mafic magmas. This magma characterized by intermediate viscosity and eruption temperatures of 1,000 °C (1,832°F). Among volcanic rocks, distinctive intermediate magmas include andesite, dacite and trachyandesite. Among plutonic rocks, distinctive intermediate magmas are diorite and granodiorite.

Diorite is a plutonic intermediate magma. Photo Credit: AlishaV

Diorite is a plutonic intermediate magma. Photo Credit: AlishaV

Ultramafic Magma

Ultramafic or ultrabasic magma yields igneous and meta-igneous rocks with smaller percentage of silica (<45% SiO2). This magma is characterized by very low viscosity and eruption temperatures of up to 1,500 °C (2,732 °F). Typical basic magmas are peridotite and comatite. The Earth’s mantle is composed of ultrabasic rocks.

Quartz (silicon dioxide) is the most widespread mineral found on the Earth’s surface. It is a key component of many felsic igneous rocks such as granite.

Quartz (silicon dioxide) is the most widespread mineral found on the Earth’s surface. It is a key component of many felsic igneous rocks such as granite.

Basic Magma

Basic or mafic magmas have 45–52% SiO2, with higher percentages of calcium, iron, and magnesium. Before cooling—due to its lower silica content, mafic lava has a lower viscosity when compared to felsic lava. It has eruption temperature of 1,300°C (2,372 °F). Water and other volatiles escape more easily from this type of magma. Eruptions of mafic-magma volcanoes are less explosive when compared to felsic-magma volcanoes. Typical basic magmas are gabbro and basalt. Most mafic-magma volcanoes are oceanic volcanoes.

Magma and Lava: Temperature

Temperature is a relatively simple variable to measure and significant amounts of data exist for different magma types. Typically, an inverse correlation exists between eruption temperature and SiO2 content of magma.

Basaltic magmas (50% SiO2) erupt at temperatures of about 1,200°C (2,192 °F), although temperatures of 1,350°C (2,462°F) were documented above Hawaiian lava lakes where volcanic gases reacted with the atmosphere.

Silicic magmas (>63% SiO2) are considerably cooler and erupt in the temperature range of 700–900°C (1,292-1,652°F).

    Basalt rocks in Iceland. Basalt is a typical basic magma with lower silica content.

Basalt rocks in Iceland. Basalt is a typical basic magma with lower silica content.

Nearly all direct temperature assessments were taken at volcanoes characterized by little or no explosive activity where geologists could securely come close to the lava. Little is known, therefore, of the temperatures of felsic lavas. Eruptions of such lavas are uncommon, and when they do happen, they are prone to be violent. The temperatures of various lava domes (most of which are round accumulations of felsic magma) have been measured remotely by an optical pyrometer. The surfaces of these rounded landforms reach temperatures of 900°C (1,652 °F). Two weeks following the eruption of Mount St. Helens in 1980, its pyroclastic flows (a destructive cloud of hot gas and rock) maintained temperatures between 300 and 420°C (572-788 °F).

Magma and Lava: Viscosity

Besides composition and temperature, magma is characterized by its resistance to flow or viscosity. Viscosity is a key property in comprehending the performance of magma and has significant influence on the manner of a volcanic eruption.

In general, predominantly mafic magmas (such as magmas that produce basalt rocks) are less viscous and hotter than predominantly silicic magmas, such as those that yield rhyolite. Low viscosity magma (flows freely and quickly) leads to less violent and milder eruptions. High viscosity magma (resistant to flow and flows slowly) leads to more violent and forceful eruptions.

Highly viscous motor oil flows promptly when hot, but becomes stiff and flows slowly when it is cold. Thus, one might infer that temperature controls the viscosity of magma and such a conjecture is somewhat correct.

Yet, we can oversimplify by saying that cool lava flows more slowly than hot lava. As a matter of fact, temperature is not the solitary control of viscosity; other factors that control viscosity include the composition, occurrence of crystals, minerals and dissolved gas.

Viscosity of magma is strongly correlated with its content of silica. Melts rich in silica are characterized by higher viscosity. Melt viscosity is also influenced by suspension of gas. Higher amount of dissolved gas (H2O, CO2) will lead to lower melt viscosity.

In felsic (silica-rich, high-viscosity) lavas, many networks of silica tetrahedra restrict flowing, since the firm bonds of the networks must be broken to let flow take place. Gases trapped in felsic lava expand as magma rises towards the surface until they blast in a forceful eruption.

Mafic lavas, on the other hand, contain fewer silica tetrahedra networks. Low viscosity of mafic magmas allows dissolved  gases to separate out and gently escape from mafic magma. A small increase in gas pressure as the mafic magma approaches the surface leads to a mild eruption.

Due to their high viscosity, felsic lavas form thick, slow-moving flows, whereas low-viscous mafic lavas have thinner flows that travel quite fast over large distances. An example of one such flow was recorded in Iceland in 1783 being 80 km (5 mi) long. Some ancient flows in the state of Washington were more than 500 km (311 mi) long.

Volcanoes and Volcanism

Volcanism refers to the processes when magma and its related gases rise through the crust of the planet and are ejected as lava onto the Earth’s surface or into the atmosphere.

Volcanoes are not distributed randomly around the planet but instead occur in well-defined zones. The most famous of these is the Pacific Ring of Fire –a clustering of volcanoes, beginning from New Zealand, proceeding through the Tonga and Solomon islands towards the Philippines. From there it winds into Japan and the Kamchatka in Russia, throughout the Aleutian islands. The ring goes on through the Cascades in Canada and the US, towards Central America. This line continues all the way down the west coast of South America to the tip of the Antarctic peninsula.

The Ring of Fire is associated with many of the largest volume and most energetic volcanic eruptions to have occurred in human history. The 1912 eruption of Katmai, the 1991 eruption of Mt. Pinatubo, the 1883 eruption of Krakatau and the 1815 eruption of Tambora– were all concentrated along the Pacific Ring of Fire.

Volcanic Eruptions and Benefits to People

Today, more than 500 volcanoes are considered active, meaning, they have erupted during historic time. Well-known instances of active volcanoes include, Mt. Etna (Sicily), Mauna Loa and Kilauea (Hawaii), Fujiyama (Japan) and Mt. St. Helens (Washington).

Despite the recorded destructiveness throughout history, volcanism and volcanoes are also known to be beneficial to people. Volcanoes produce fertile soil, valuable minerals and useful geothermal energy. Fertile soil of volcanoes contains numerous minerals that farmers can benefit from by cultivating crops and collecting rich harvests. Volcanoes are also useful for minerals and the precious stones they accumulate. Lava in volcanoes crystallizes to form minerals that, depending on the composition, form gold, silver, diamond, copper and zinc. Volcanoes are useful in producing geothermal energy. By harnessing the steam from underground water heated by the Earth’s magma, geothermal energy can be used by geothermal power stations to produce electricity for home and industrial use.


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